Rotary pump with displacement control

ABSTRACT

An axial piston pump including a rotary cylinder block with reciprocable pistons controlled by an adjustable swashplate for varying displacement as the cylinder block rotates against a valve plate, together with a control port in the valve plate for supplying fluid under pressure to the pumping pistons for controlling the position of the swashplate and therefore the displacement through the medium of the pumping pistons rather than separate control means.

United States Patent 91 Reynolds [45] Apr. 17, 1973 I I RQTARY PUMP WITHFOREIGN PATENTS OR APPLICATIONS DISPLACEMENT CONTROL 463,045 4/1951Italy ..91/505 [75] Inventor: Richard W. Reynolds, Rockford, Ill. [73]Assignee: Sundstrand Corporation Rockford Primary Examiner-William Freshm Assistant ExaminerRichard E. Gluck Att0rneyHofgren, Wegner, Allen,Stellman & Mc-

22 Filed: Apr. 12, 1971 Cord Appl, No.: 133,278

[52] US. Cl. ..91/475, 91/505, 417/222 [51] Int. Cl ..F0lb 1/00, FOlb3/00, F04b H26 [58] Field of Search ..91/505, 506, 487, 91/475, 6.5,599; 417/222 [56] References Cited UNITED STATES PATENTS 3,190,2326/1965 Budzich ..417/222 3,199,461 8/1965 Wolf 91/506 X 3,230,894 H1966Badenoch et al. ..9l/6.5 X

[57] ABSTRACT An axial piston pump including a rotary cylinder blockwith reciprocable pistons controlled by an adjustable swashplate forvarying displacement as the cylinder block rotates against a valveplate, together with a control port in the valve plate for supplyingfluid under pressure to the pumping pistons for controlling the positionof the swashplate and therefore the displacement through the medium ofthe pumping pistons rather than separate control means.

13 Claims, 2 Drawing Figures PATENTEB APR 1 7 I975 INVENTOR- RICHARD W.REYNOLDS {964%, M a fl/M'w- M mba awkw- ATTORNEYS.

ROTARY PUMP WITH DISPLACEMENT CONTROL BACKGROUND OF THE INVENTION Rotaryhydraulic fluid translating devices such as pumps and motors arecommonplace in the art and comprise both axial piston devices and radialdevices involving pistons or vanes. In general, an axial piston device,for example, may include a housing comprised of two or more housingportions suitably joined, a cylinder block rotatable in the housing withaxially disposed pistons reciprocable in fluid chambers under control ofa cam or swashplate as the cylinder block rotates against a valve platehaving inlet and outlet ports communicable serially with the fluidchambers. It is also common practice to provide for variabledisplacement of such hydraulic units by pivotally mounting theswashplate for adjustment about an axis generally transverse to thecylinder block axis in a manner to vary the stroke of the pistons.Frequently, automatic controls are desirable in variable displacementhydraulic units. Controls may include a constant biasing means acting onthe displacement varying member in a manner to urge it into stroke orout of stroke, and a variable bias control may be applicable inopposition to the constant bias in order to overbalance or underbalancethe latter selectively in a manner to vary displacement according tosome desired principle. In particular, in axial piston pumps, forexample, a constant pressure control may utilize a spring biasing forceurging the swashplate into stroke, and a piston and cylinder device forurging the swashplate out of stroke under control of a valve responsiveto discharge pressure in a manner effective to maintain constantdischarge pressure.

In devices of the character described, the control piston and cylinderdevice and the linkages associated with it often require precisemanufacture and often are fragile so that the control becomes a criticalfactor in the unit controlling both the cost of manufacture and thedurability. Further, in order to conduct control pressure from the pumpoutlet to the control valve, and the control piston and cylinder device,the porting must pass through one or more joints at the juncture ofhousing members which gives rise to potential leakage problems.

SUMMARY OF THE INVENTION The present invention has for its generalobject a simplification of control means in variable displacementhydraulic units by utilizing the main pistons to control displacement.

More particularly, the variable displacement cam controlling the strokeof the pistons is subjected to a displacement varying force byapplication of control fluid to the main pistons, thereby eliminatingthe need for a separate control piston and cylinder device of expensiveand fragile construction.

In a preferred embodiment illustrated herein, the principles of theinvention are demonstrated in connection with an axial piston pump witha cam or swashplate mounted for pivotal adjustment about an axisgenerally transverse to the axis of the rotary cylinder block. As thecylinder block rotates, axial pistons are reciprocated by the swashplateat one end of the cylinder block, while the piston chambers successivelycommunicate with inlet and outlet ports in a valve plate at the oppositeend of the cylinder block. In order to control the pivotal adjustment ofthe swashplate, the valve plate includes a control port located betweenthe inlet and outlet ports and adapted to supply control fluidsuccessively to the pistons as they pass the control port, thereby toapply a force for varying the angle of the swashplate.

In the control shown, the swashplate is biased in a stroke increasingdirection and the control port is utilized to supply fluid for biasingthe swashplate in a stroke reducing direction. Fluid is supplied to thecontrol port under control of a valve responsive to outlet pressure andadapted to function in a way to control the swashplate to maintain theoutlet pressure constant. Preferably, the control port is centrallylocated between the outlet port and the inlet port and spaced from eachby an amount approximately equal the length of each cylinder block portso that the control port is always in communication with one of thecylinder ports, and the size of the control port is such that itprimarily communicates with only one cylinder port.

As illustrated herein, the inlet and outlet ports in the valve platehave a greater length at one side of the swashplate pivot axis in orderto utilize the pumping pistons to urge the swashplate into stroke.However, other means may be used for urging the swashplate into stroke.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is a diagrammatic illustrationof a pump together with control circuit embodying the principles of thepresent invention; and

FIG. 2 is a sectional view taken at about the line 22 in FIG. 1, showingthe arrangement of the control port in the valve plate.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENT Referring now to thedrawings in more detail, the invention is illustrated in connection witha pump generally designated 10 which may be of the type described in USPat. Nos. 3,366,072 and 3,366,968, if desired, but the invention is notlimited to use with such pump constructions. In FIG. I, the pump isillustrated diagrammatically as including a rotatable cylinder blockappropriately mounted in bearings as at 12 for rotation about alongitudinal central axis 13. The cylinder block is formed with anannular series of axially disposed fluid chambers 15 arranged around theaxis of rotation and each including a reciprocable piston as at 17. Inorder to control reciprocation of the pistons on rotation of thecylinder block, there is an adjustable cam or swashplate 18 mounted forpivotal movement about an axis 19 transverse to cylinder block axis 13.To facilitate pivotal adjustment, the swashplate is formed with one ormore arcuate bearing surfaces as at 20 supported on roller bearings 21in an arcuately curved bearing race 22. In the pump shown, there areseven fluid chambers 15 with pistons 17, but there may be other numbers.

In the preferred construction illustrated, the face of the swashplate 18at 25 supports an annular bearing ring 26. The bearing ring includes aplurality of sockets for universally receiving the spherical ends 28 ofa plurality of piston rods 29 having opposite spherical ends 30universally mounted in the pistons 17 respectively. Attached to thebearing ring 26 is a timing member 32 which allows limited pivotalmovement of the connecting rods 29 but essentially maintains the rodsgenerally perpendicular to the bearing ring so that as the cylinderblock rotates, the bearing ring rotates with the block. The timingmember 32 is piloted on a spherically shaped member 34 appropriatelymounted on the cylinder block 1 1, all as described in more detail inthe aforementioned U. S. Pat. Nos. 3,366,072 and 3,366,968.

At the opposite end of the cylinder block from the swashplate 18, thecylinder block rotates against the face of a stationary valve plate 36appropriately mounted in a suitable housing with the remaining pumpstructure and including ports for supplying fluid to and from the pumpchambers through cylinder ports 37 which lead from the chambers 15 tothe end of the cylinder block. As seen in FIG. 2, the port plate 36includes an arcuate inlet port 38 and a similarly shaped arcuate outletport 39 symmetrically disposed in the plate at opposite sides of apiston dead center axis 40 which extends generally transverse to thecylinder block axis and transverse to the swashplate pivot axis and liesin the plane passing through the angular positions where the pistonsreach opposite ends of their strokes.

In operation of the pump as described thus far, it will be understoodthat when the swashplate 18 is pivoted as shown in FIG. 1, the pistons17 reciprocate as the cylinder block is rotated, and if inlet fluid atlow pressure is supplied to the arcuate inlet port 38, it is admitted tothe passing cylinder ports 37 as the pistons are withdrawn from thecylinders 15. As the cylinder ports 37 pass the high pressure dischargeport 39, the pistons are forced into the cylinders and fluid is pumpedout of the discharge port under pressure. The displacement of the unit,that is, the volume of fluid displaced on each revolution of thecylinder block, may be varied by pivoting the swashplate 18 to increaseor decrease the angle of the swashplate relative to the end of thecylinder block. If the face of the swashplate is parallel to the end ofthe cylinder block, the pistons will not reciprocate and thedisplacement is zero.

In the construction as illustrated, the swashplate 18 is mounted topivot about an axis (19) which is transverse to the cylinder block axisand which intersects the cylinder block axis perpendicular thereto andperpendicular to a timing axis coincident with the piston dead centeraxis 40 and centrally disposed between the inlet and outlet ports 38 and39. The swashplate is biased in a stroke increasing direction byarrangement of the arcuate ports 38 and 39 with a greater length abovethe pivot axis 19 than below, so that the pumping pistons exposed tohigh pressure in the port 39 exert a greater force on the swashplateabove the axis 19 than below the axis 19. By such an arrangement, theswashplate is normally biased into stroke by use of the main pumpingpistons without other additional structure, as a result of which thepump is simplified, the cost is reduced, and life is prolonged. Ifdesired, other means may be employed separately or in combination withthe arrangement described for purposes of biasing the swashplate intostroke. For example, the swashplate pivot axis 19 may be lowered fromthe transverse center line through the port plate, and such arrangementwill have the effect of increasing the length of the arcuate port 39above the pivot axis in a way such that the pumping pistons exert astroke increasing influence on the swashplate. As another example, theswashplate may be urged in a stroke increasing direction through themedium ofa suitable spring mechanism applying a constant bias along thelines of that illustrated in the aforementioned U. S. Pat. Nos.3,366,072 and 3,366,968.

In order to apply a force to the swashplate in a stroke decreasingdirection, to oppose the stroke increasing force and control theswashplate angle selectively, the present invention contemplatesapplication of a control force through the medium of the main pumpingpistons by subjecting the latter to control fluid under pressure. Forsuch purposes, the port plate 36 is formed with a control port 42located in the path of the cylinder ports 37 and located centrallybetween the end 39a of the outlet port 39 and the beginning 38a of theinlet port 38. As viewed in FIG. 2, when the device is employed asapump, the cylinder block rotates counterclockwise in the direction ofthe arrow 43. Thus, it will be seen that as a pump cylinder 15 passesthe control port 42, the piston 17 therein may be subjected to controlfluid under pressure to apply a stroke decreasing force. Preferably, thecontrol port is spaced from the adjacent ends of the ports 38 and 39 byan amount equal to the length of cylinder ports 37 so that one port 37is substantially always exposed to control fluid under pressure and onlyone at a time is exposed. The use of the main pumping pistons forbiasing the swashplate in a stroke reducing direction simplifies thepump construction in avoiding the need for additional separate controlmeans which traditionally has required precise construction involvingsubstantial expense and limited life. Use of the main pumping pistonstakes advantage of pump parts which are normally constructed for heavyduty performance.

In the circuit illustrated in FIG. 1, a supply of fluid under pressureto the control port 42 is controlled by a constant pressure controlvalve as at which may be of the type described in US. Pat. No. 3,465,680if desired. In order to provide for maintaining the pump at a zerostroke or substantially reduced stroke near zero, the circuit mayinclude a solenoid depressurization valve as at 51 which may be of thetype described in U. S. Pat. No. 3,563,675.

As shown in FIG. 1, the pump outlet port 39 communicates with adischarge conduit 53, and branch conduit 54 communicates dischargepressure to the constant pressure control valve 50. The latter includesa valve member 55 having enlarged lands 56 and 57 with a reduced stemportion 58 therebetween. The end of the valve member 55 is formed with alarge disc 59 which provides a spring seat for a coil spring 61 in aspring chamber 62 where the spring is grounded at the end of thechamber. The valve 50 includes a port 66 normally closed by land 56 anda port 67 normally closed by land 57 when the spring 61 biases the valvemember toward the left as viewed in FIG. 1. The ports 66 and 67communicate with a passage 68 leading to the control port 42. When thevalve member 55 is positioned as illustrated, the reduced stem portion58 is aligned with a drain port 69.

In operation, the swashplate is normally biased into stroke as describedabove so that on rotation of the cylinder block, the pump delivers fluidunder pressure to the discharge conduit 53. The valve 50 sensesdischarge pressure through the conduit 54, and when the pressure risesunduly, the valve stem 55 is moved to the right against the bias ofspring 61, admitting fluid under pressure past the land 57 to the port67 and the control port 42 to reduce pump displacement. In order to dampthe action of the pressure responsive valve as described in U. S. Pat.No. 3,465,680, movement of the valve member 55 toward the right alsocommunicates port 66 with drain port 69 through the reduced stem portion58 in a way to reduce the effect of the correction called for by thepressure response of the valve. When the discharge pressure is reducedappropriately as a result of the reduction of the swashplate angle, thespring 61 is again effective to move the valve member toward the leftwhere the pressure passage 64 is disconnected from the control port 42,and the stroke may be increased. In this manner, constant pressure maybe maintained in the discharge conduit 53.

The depressurization valve 51 includes a valve stem 70 with a reducedstern portion 71 between enlarged portions 72 and 73. The valve memberis normally positioned as illustrated in FIG. 1 so that the enlargedportion 72 blocks a high pressure passage 54a and communicates a controlport 74 with a drain port 75 leading to a conduit 76 which in turnconnects with an inlet conduit 77 leading to the pump inlet port 38.When it is desired to take the pump out of stroke, the valve stem 70 maybe moved to the right as viewed in FIG. 1 by means such as a selectivelyenergizable solenoid to a position where the drain port 75 is blocked bythe enlarged stem portion 73, and the pressure port 54a is communicatedwith the control port 74 through the reduced stem portion 71. At thattime, discharge pressure is supplied to both sides of the control land57 in the constant pressure valve and the valve functions only todeliver discharge pressure to the control port 42, without thecapability of connecting the latter to drain, so that the effect is toapply a constant bias to the swashplate in a stroke reducing direction.The latter operation is described in detail in the aforementioned U. S.Pat. No. 3,563,675.

It will be appreciated that the control of displacement through themedium of one of the main pistons in the hydraulic unit represents agreat simplification in contrast to provision of separate controlmechanisms. While the invention has been illustrated in connection withan axial piston swashplate pump, it should be understood that theprinciples of the invention are also applicable to other hydraulic unitssuch as radial piston and vane units. Also, while the inlet port 38 andthe outlet port 39 have been illustrated as symmetrical relative to thepiston dead center plane 40 and the timing axis coincident therewith, itis conventional practice to shorten the beginning of the outlet port at39b in order to precompress the fluid in each cylinder by pistonmovement before exposure to the high pressure port 39, and suchconstruction may be utilized herein.

In the port plate illustrated in FIG. 2, with the control port 42located at top dead center in the piston dead center plane, where thepistons begin the intake stroke, there may be some tendency to lose someof the effect of the control pressure due to the intake stroke enlargingthe fluid chamber, but in actual practice in a unit of the typedescribed, it has been found that the inertia of the system is such thatthe control port is substantially always under pressure withoutsignificant suction effect. However, if desired, in order to ensurepressure effect in the control port 42, the latter may be indexed a fewdegrees clockwise, so that the port is entirely located at the side ofthe piston dead center plane where the pistons are cammed in a directionreducing the volume of the fluid chambers. In such event, the end ofdischarge port 39 may be shortened at 39a and the beginning of inletport 38 may be lengthened at 3811 so that the control port is centrallydisposed between the port ends and spaced therefrom by an amountapproximately corresponding to the length of cylinder ports 37.

Iclaim:

l. A rotary fluid translating apparatus, comprising, a rotary cylinderblock having a plurality of fluid chambers angularly spaced around theaxis of rotation of the block, a plurality of pistons reciprocable inthe fluid chambers, a cam disposed relative to the cylinder block forcausing reciprocation of the pistons as the cylinder block rotates,means mounting the cam for adjustment to vary the stroke of the pistons,means biasing the cam in one direction, a port plate adjacent thecylinder block having inlet and outlet ports, ports in the cylinderblock communicating the fluid chambers successively with the inlet andoutlet ports as the cylinder block rotates, a control port in the portplate for supplying fluid under pressure successively to the fluidchambers as the cylinder block rotates to apply a force through thepistons to the cam in opposition to the biasing means for controllingthe position of the. cam, and selectively operable means for supplyingcontrol fluid under pressure to the control port.

2. A rotary fluid translating apparatus as defined in claim 1, includingselectively operable valve means for controlling the supply of controlfluid to the control port.

3. A rotary fluid translating apparatus as defined in claim 1, includingmeans biasing the cam in a direction to cause the pistons to stroke whenthe cylinder block rotates, and means locating the control port tosupply fluid under pressure to the pistons while positioned to bias thecam in a stroke reducing direction.

4. A rotary fluid translating apparatus as defined in claim 3, includingvalve means responsive to outlet pressure for controlling the supply ofcontrol fluid to the control port.

5. An axial piston pump, comprising, a rotatable cylinder block havingan annular series of axially disposed fluid chambers spaced around theaxis of the cylinder block, piston means reciprocable in the fluidchambers and projecting from one end of the cylinder block, a camdisposed adjacent said one end of the cylinder block for causingreciprocation of the piston means as the cylinder block rotates, a valveplate engaging the other end of the cylinder block, inlet and outletports in the valve plate for admitting low pressure fluid and exhaustinghigh pressure fluid, ports in the end of the cylinder blockcommunicating the chambers successively with the inlet and outlet portsas the cylinder block rotates, means mounting the cam for pivotaladjustment to vary the stroking of the piston means as the cylinderblock rotates, means biasing the cam in a direction to increase strokingof the piston means, and a control port in the valve plate between theoutlet port and the inlet port for successive communication with thechambers as the block rotates to supply control fluid to successivepiston means to bias the cam in a stroke reducing direction.

6. A pump as defined in claim including conduit means for supplyingcontrol fluid under pressure to the control port, and selectivelyoperable valve means in the conduit means for controlling the flow ofcontrol fluid to the control port.

7. A pump as defined in claim 6 wherein the valve means is responsive toincrease in outlet pressure to reduce displacement.

8. An axial piston pump, comprising, a rotatable cylinder block havingan annular series of fluid chambers spaced around the axis of thecylinder block, piston means reciprocable in the fluid chambers, aswashplate disposed adjacent one end of the cylinder block for causingreciprocation of the pistons as the cylinder block rotates, meansmounting the swashplate for pivotal adjustment about an axis generallytransverse to the axis of the cylinder block to vary the stroke thecylinder block, arcuate inlet and outlet ports in the valve plate atopposite sides ofa piston dead center axis generally transverse to thecylinder block axis and the swashplate pivot axis for admitting andexhausting fluid, ports in the end of the cylinder block communicatingthe chambers successively with the inlet and outlet ports as thecylinder block rotates, said arcuate inlet and outlet ports including agreater length at one side of the swashplate pivot axis so that theswashplate is biased by the pistons in a direction to increase pistonstroke, and a control port in the valve plate between the outlet portand the inlet port at the other side of the swashplate pivot axis forsuccessive communication with the chambers as the block rotates tosupply control fluid to successive pistons to bias the swashplate in astroke reducing direction.

9. A pump as defined in claim 8 wherein the control port is locatedcentrally between adjacent ends of the outlet port and inlet port.

10. A pump as defined in claim 9 wherein the space of the pistons, avalve plate engaging the other end of between the control port and eachof the inlet and outlet ports corresponds to the length of each cylinderport.

11. An axial piston pump, comprising, a rotatable cylinder block havingan annular series of axially disposed fluid chambers spaced around theaxis of the cylinder block, piston means reciprocable in the fluidchambers and projecting from one end of the cylinder block, a swashplatedisposed adjacent said one end of the cylinder block for causingreciprocation of the piston means as the cylinder block rotates, a valveplate engaging the other end of the cylinder block, arcuate inlet andoutlet ports in the valve plate for admitting and exhausting fluid,ports in the end of the cylinder block communicating the chamberssuccessively with the inlet and outlet ports as the cylinder blockrotates, means mounting the swashplate for pivotal movement about anaxis generally transverse to the axis of rotation of the cylinder block,means biasing the swashplate in a direction to cause stroking of thepiston means as the cylinder block rotates, a control port in the valveplate between the inlet and outlet ports for successive communicationwith the chambers as the block rotates, conduit means for supplyingcontrol fluid to the control port to supply fluid to successive pistonsas the cylinder block rotates thereby to bias the swashplate in a strokereducing direction, and valve means controlling the supply of controlfluid to the control port responsive to outlet pressure so thatincreases in outlet pressure reduce displacement and constant outletpressure is maintained.

12. A pump as defined in claim 11 wherein the inlet and outlet ports aresymmetrically disposed at opposite sides of a piston dead center axisgenerally transverse to the cylinder block axis and the swashplate pivotaxis and include a greater length at one side of the swashplate pivotaxis opposite from the control port so that the pistons bias theswashplate in a stroke increasing direction.

13. A pump as defined in claim 12 wherein the control port is centrallylocated between adjacent ends of the outlet port and inlet port andspaced therefrom by an amount corresponding to the length of eachcylinder port.

1. A rotary fluid translating apparatus, comprising, a rotary cylinderblock having a plurality of fluid chambers angularly spaced around theaxis of rotation of the block, a plurality of pistons reciprocable inthe fluid chambers, a cam disposed relative to the cylinder block forcausing reciprocation of the pistons as the cylinder block rotates,means mounting the cam for adjustment to vary the stroke of the pistons,means biasing the cam in one direction, a port plate adjacent thecylinder block having inlet and outlet ports, ports in the cylinderblock communicating the fluid chambers successively with the inlet andoutlet ports as the cylinder block rotates, a control port in the portplate for supplying fluid under pressure successively to the fluidchambers as the cylinder block rotates to apply a force through thepistons to the cam in opposition to the biasing means for controllingthe position of the cam, and selectively operable means for supplyingcontrol fluid under pressure to the control port.
 2. A rotary fluidtranslating apparatus as defined in claim 1, including selectivelyoperable valve means for controlling the supply of control fluid to thecontrol port.
 3. A rotary fluid translating apparatus as defined inclaim 1, including means biasing the cam in a direction to cause thepistons to Stroke when the cylinder block rotates, and means locatingthe control port to supply fluid under pressure to the pistons whilepositioned to bias the cam in a stroke reducing direction.
 4. A rotaryfluid translating apparatus as defined in claim 3, including valve meansresponsive to outlet pressure for controlling the supply of controlfluid to the control port.
 5. An axial piston pump, comprising, arotatable cylinder block having an annular series of axially disposedfluid chambers spaced around the axis of the cylinder block, pistonmeans reciprocable in the fluid chambers and projecting from one end ofthe cylinder block, a cam disposed adjacent said one end of the cylinderblock for causing reciprocation of the piston means as the cylinderblock rotates, a valve plate engaging the other end of the cylinderblock, inlet and outlet ports in the valve plate for admitting lowpressure fluid and exhausting high pressure fluid, ports in the end ofthe cylinder block communicating the chambers successively with theinlet and outlet ports as the cylinder block rotates, means mounting thecam for pivotal adjustment to vary the stroking of the piston means asthe cylinder block rotates, means biasing the cam in a direction toincrease stroking of the piston means, and a control port in the valveplate between the outlet port and the inlet port for successivecommunication with the chambers as the block rotates to supply controlfluid to successive piston means to bias the cam in a stroke reducingdirection.
 6. A pump as defined in claim 5 including conduit means forsupplying control fluid under pressure to the control port, andselectively operable valve means in the conduit means for controllingthe flow of control fluid to the control port.
 7. A pump as defined inclaim 6 wherein the valve means is responsive to increase in outletpressure to reduce displacement.
 8. An axial piston pump, comprising, arotatable cylinder block having an annular series of fluid chambersspaced around the axis of the cylinder block, piston means reciprocablein the fluid chambers, a swashplate disposed adjacent one end of thecylinder block for causing reciprocation of the pistons as the cylinderblock rotates, means mounting the swashplate for pivotal adjustmentabout an axis generally transverse to the axis of the cylinder block tovary the stroke of the pistons, a valve plate engaging the other end ofthe cylinder block, arcuate inlet and outlet ports in the valve plate atopposite sides of a piston dead center axis generally transverse to thecylinder block axis and the swashplate pivot axis for admitting andexhausting fluid, ports in the end of the cylinder block communicatingthe chambers successively with the inlet and outlet ports as thecylinder block rotates, said arcuate inlet and outlet ports including agreater length at one side of the swashplate pivot axis so that theswashplate is biased by the pistons in a direction to increase pistonstroke, and a control port in the valve plate between the outlet portand the inlet port at the other side of the swashplate pivot axis forsuccessive communication with the chambers as the block rotates tosupply control fluid to successive pistons to bias the swashplate in astroke reducing direction.
 9. A pump as defined in claim 8 wherein thecontrol port is located centrally between adjacent ends of the outletport and inlet port.
 10. A pump as defined in claim 9 wherein the spacebetween the control port and each of the inlet and outlet portscorresponds to the length of each cylinder port.
 11. An axial pistonpump, comprising, a rotatable cylinder block having an annular series ofaxially disposed fluid chambers spaced around the axis of the cylinderblock, piston means reciprocable in the fluid chambers and projectingfrom one end of the cylinder block, a swashplate disposed adjacent saidone end of the cylinder block for causing reciprocation of the pistonmeans as the cylinder block rotates, a valve plate Engaging the otherend of the cylinder block, arcuate inlet and outlet ports in the valveplate for admitting and exhausting fluid, ports in the end of thecylinder block communicating the chambers successively with the inletand outlet ports as the cylinder block rotates, means mounting theswashplate for pivotal movement about an axis generally transverse tothe axis of rotation of the cylinder block, means biasing the swashplatein a direction to cause stroking of the piston means as the cylinderblock rotates, a control port in the valve plate between the inlet andoutlet ports for successive communication with the chambers as the blockrotates, conduit means for supplying control fluid to the control portto supply fluid to successive pistons as the cylinder block rotatesthereby to bias the swashplate in a stroke reducing direction, and valvemeans controlling the supply of control fluid to the control portresponsive to outlet pressure so that increases in outlet pressurereduce displacement and constant outlet pressure is maintained.
 12. Apump as defined in claim 11 wherein the inlet and outlet ports aresymmetrically disposed at opposite sides of a piston dead center axisgenerally transverse to the cylinder block axis and the swashplate pivotaxis and include a greater length at one side of the swashplate pivotaxis opposite from the control port so that the pistons bias theswashplate in a stroke increasing direction.
 13. A pump as defined inclaim 12 wherein the control port is centrally located between adjacentends of the outlet port and inlet port and spaced therefrom by an amountcorresponding to the length of each cylinder port.